Ionic Liquids in Green Carbonate Synthesis

Abstract

Ionic liquids (ILs), made of relatively large organic cations and inorganic anions, could contribute as solvents and catalysts to green organic synthetic reactions [1-5]. The reaction systems using no organic solvents, dense phase carbon dioxide (supercritical and carbon dioxide-dissolved expanded liquid phases), and/or water are obviously advantageous from environmental viewpoints [1]. ILs would be one of attractive components for environmentfriendly reaction and separation processes because of their physicochemical properties. But we should also note the possibility of negative effects caused by their toxicity on environment and products when we use ILs for practical applications. There are a number of ILs using different cations and anions, and a few typical examples are shown in Fig. 1. It is evidenced that ILs act as good catalysts for several organic synthetic reactions, as noted in recent review articles [1–5], including Friedel-Crafts reactions, Diels-Alder reactions, hydrogenation, polymerization, and other reactions. Haumann and Riisager discuss various aspects (structure of substrates, type of ILs, form of ILs catalysts) in hydroformylation reactions using ILs [4]. Recently Baiker et al. have published a review on multiphase catalytic systems including ILs and carbon dioxide [5]. They point out the significance of the combination of ILs and carbon dioxide for chemical and separation processes. This review deals with physicochemical features of those multiphase reaction systems and catalytic reactions therein including those using carbon dioxide as a reactant. After considering those previous reviews and other chapters of this book, we will concentrate our attention on the application of ILs to the synthesis of carbonate and related compounds using carbon dioxide. Those compounds are of practical importance as solvents, reagents, fuel additives, and intermediates in the production of pharmaceuticals and fine chemicals [6,7]. The authors previously reviewed the potential application of ILs for the synthesis of cyclic carbonates from carbon dioxide [8]. The present chapter will deal with the synthesis of cyclic carbonates, dialkyl carbonates, other related compounds, and polycarbonates using ILs as solvents and catalysts.